Fluid power systems require an exceptionally diverse range of control valves for the various tasks these systems perform. Fluid power systems are found in transportation vehicles, industrial facilities, defence systems, residential and commercial buildings. Applications range from a car jack to spacecraft guidance systems.
Fluid control valves are required for many different functions in these diverse systems. These functions include: actuation, venting, relief, pressure control, fluid counter balance, fluid braking, flow control, pressure reducing, blocking and sequencing. Single valves are also required to provide multiple functions with multiple ports.
The larger and more demanding applications generally require pilot operated control valves. The pilot valves provide small amounts of fluid at precisely controlled conditions, which then actuates and controls larger valves to provide larger amounts of precisely controlled fluid for the application.
Typically pilot operated fluid control valve has a spool piece which is displaced small distances by fluid pressures on one or both ends supplied by one or two pilot valves. Displacement of the spool piece controls fluid flow and/or pressure from one or more main fluid ports to other main fluid port(s). Displacement of the spool piece may also be affected by springs or other forces. A balance of fluid (at pilot and main ports) pressures acting against areas of spool piece creates a force, which when combined with spring or other forces against on the spool piece, determines it's position or movement with respect to restriction of one or more main ports.
Pilot operated fluid control valves may incorporate other features. Strainers or filters may be included to minimize the possibility of clogging of orifices or moving parts. Spool may be sectioned to provide adjustment or delay for multiport valves. Multiple independent spool pieces have also been combined in one valve body.
However, prior art control valves are not capable of major alterations of function without modifications. Pressure balance and forces on the spool piece can be varied, but basic operation remains unchanged in prior art valves. This is especially true of three-way and four-way multiport valves. A prior art three-way valve which doesn't provide a flow path between two ports, can not be adjusted to provide this connection. Spool piece segments could also not be actuated in simultaneous opposite directions for different ports. Thus, spool piece motion in prior art valves fixed a relationship between all ports which could not be altered without valve port or spool piece modification.
The inability to vary relationships among prior art fluid control valves has resulted in a proliferation of specialty multi-port valves to perform the wide variety of control functions required. This can create large inventories of spare parts required for large/complex fluid power systems. Economies of mass production are also lost for each specialized valve.
The inability to vary relationships has also resulted in multiple valves for single fluid control points. In order to achieve sufficient control, flapper valves have been used downstream of three-way or four-way valves to provide variable orifice control for multiport applications.
Leakage has also been a problem in some prior art valves, especially closed center three-way valves. Stroke length and other limitations precluded sufficient contact length to reliably seal, causing leakage in the closed position.